The present invention relates to a coating apparatus for coating moving webs, particularly webs of paper or cardboard, of the type in which a scraper element mounted on a scraper holder which extends transversely to the direction of travel of the web is used to scrape excess coating material off the web. In such apparatus, a stop determines the position of the scraper element relative to the web when it is operating to clean the excess coating material off the web. Positioning means are provided to move the scraper element between an operating position, in which the scraper element is pressed against the web, and a rest position, in which the scraper element is spaced from the web.
In the operation of such coating apparatus, it is important to obtain a layer of coating material which is as uniform in thickness as possible. The web can travel over a rotatable roller or over a stationary supporting device at the place where the excess coating composition is scraped off, or the web can travel between two symmetrically arranged scraper devices if it has been previously coated on both sides.
A coating apparatus of this kind is known, for instance, from the journal "Wochenblatt fuer Papierfabrikation", issue No. 16 (1978), pages 619 to 624. In that article, the scraper element (preferably a doctor blade) is moved relative to the scraper holder by means of an inflatable pressure hose. The scraper holder, which rests on two swing levers, can be moved by hydraulic cylinders from a position of rest in which it is spaced from the web toward the web until it is in the operating position, which is determined by stops. In this position the scraper element is initially stopped at a slight distance, for instance 1 mm, from the web which is to be coated. The scraper element is then applied to the coating material present on the traveling web by means of the inflation of the pressure hose. The latter presses the scraper element elastically against the web which is to be coated, the elasticity being present both in the direction towards the web and in the opposite direction (away from the web). This so-called "elastic travel" is desirable in most cases. The force with which the scraper element is pressed against the web can be varied by changing the pressure in the pressure hose.
For certain types of paper or cardboard or for certain coating materials, however, so-called "rigid travel" is desired. In a rigid travel arrangement, the scraper element is held rigidly in position relative to the scraper holder, regardless of variations in thickness of the web or of the layer of coating material applied thereto, and regardless of variations in the consistency of the coating material. However, a completely rigid attachment of the scraper element would result in the danger of the coating apparatus being damaged in the case of certain malfunctions in the operation of the apparatus, especially in the event of the web wrapping itself in several layers around the counter-roller.